The present invention relates to a method for coordinate measurement of workpieces, using a probe head of the switching type to produce a signal upon contact with a workpiece to be measured, and to a coordinate-measuring machine suitable for carrying out the method.
Switching-type probe heads in use today are of two varieties. In one of these varieties, probe contact with the workpiece produces a switch signal, upon dislodging the normally seated position of a movable probe pin, thereby breaking an electrical-circuit through seat elements. In another variety, one or more highly sensitive piezoelectric elements in or near the probe pin produce an electrical pulse signal at the instant of workpiece contact.
It is also known, in a switching-type probe head, to provide for two different signals to be produced in the course of a given work-contacting procedure, namely, a piezoelectric signal plus an ensuing switch signal upon probe-pin dislodgement from its normally seated position. The respective signals from these two actions are somewhat staggered in time, since the seating switch opens later than the instant of sensitive piezoelectric-crystal excitation. Such probe heads are illustratively described in German OS 3,831,974 and in U.S. Pat. No. 4,177,568.
In order to determine the coordinates of the point of probe contact with the workpiece with such a two-signal switching-type probe head, it has to date been necessary to proceed as follows:
The scale-reading counter values of the respective coordinate-measuring systems (scale systems) of the coordinate-measuring machine are entered into intermediate storage when, for example, the piezoelectric signal intensity exceeds a predetermined trigger threshold. And if, within a predetermined ensuing period of 200 ms, the later seating-switch signal were to reach the control means of the machine, the piezo signal would be recognized as certifying a valid work-contact event, and the values that had been entered into intermediate storage would be entered into computer storage as the coordinate-measurement values. On the other hand, failure to produce a seating-switch signal within the predetermined period was cause for an automatic repeat of the measurement procedure, on the assumption (by the control means of the machine) that the piezoelectric signal was not an indication of a workpiece contact, but was for example a so-called "air contact" caused by noise or vibration, from the environment or from a bearing or a drive of the machine.
U.S. Pat. No. 4,455,755 also describes a method which is very similar to the one described above.
Although the known method is satisfactory in many situations, there are situations in which use of the method can lead to invalid measurements. If, for example, measurements are made on parts having a rough aluminum surface or, in general, workpieces are measured at slow speeds of workpiece contact, the signal of the piezoelectric element may not be produced, or it may be lost in the noise level. In such cases, even a repeat of the measurement does not produce a different result, and one must either increase the speed of workpiece contact, or the electromechanical switching signal produced by probe dislodgement from seated position must by itself assume the function of developing the work-contact signal. However, to rely on the seating switch alone is to lose precision measurement, since the seating-switch signal occurs later than the signal of the piezoelectric element and thus is no longer indicative of the time to establish exact contact coordinates of the point of workpiece contact.